U.S. patent number 7,002,517 [Application Number 10/600,293] was granted by the patent office on 2006-02-21 for fixed-frequency beam-steerable leaky-wave microstrip antenna.
This patent grant is currently assigned to Anritsu Company. Invention is credited to Karam Michael Noujeim.
United States Patent |
7,002,517 |
Noujeim |
February 21, 2006 |
Fixed-frequency beam-steerable leaky-wave microstrip antenna
Abstract
A fixed frequency continuously beam-steerable leaky-wave antenna
in microstrip is disclosed. The antenna's radiating strips are
loaded with identical shunt-mounted variable-reactance elements,
resulting in low reverse-bias-voltage requirements. By varying the
reverse-bias voltage across the variable-reactance elements, the
main beam of the antenna may be scanned continuously at fixed
frequency. The antenna may consist of an array of radiating strips,
wherein each strip includes a variable-reactance element. Changing
the element's reactance value has a similar effect as changing the
length of the radiating strips. This is accompanied by a change in
the phase velocity of the electromagnetic wave traveling along the
antenna, and results in continuous fixed-frequency main-beam
steering. Alternatively, the antenna may consist of two long
radiating strips separated by a small gap, wherein identical
variable-reactance elements are mounted in shunt across the gap at
regular intervals. A continuous change in the reactance value has a
similar effect as changing continuously the width of the radiating
strips. This results in a continuous change in the phase velocity
of the electromagnetic wave traveling along the antenna, thereby
achieving continuous fixed-frequency main-beam steering.
Inventors: |
Noujeim; Karam Michael
(Sunnyvale, CA) |
Assignee: |
Anritsu Company (Morgan Hill,
CA)
|
Family
ID: |
34062216 |
Appl.
No.: |
10/600,293 |
Filed: |
June 20, 2003 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20050012667 A1 |
Jan 20, 2005 |
|
Current U.S.
Class: |
343/700MS;
343/745; 343/754 |
Current CPC
Class: |
H01Q
1/38 (20130101); H01Q 13/20 (20130101); H01Q
13/206 (20130101); H01Q 23/00 (20130101) |
Current International
Class: |
H01Q
1/38 (20060101) |
Field of
Search: |
;343/700MS,745,749,754,757,824,860 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
I J. Bahl and Prakash Bhartia, Leaky-Wave Antennas Using Artificial
Dielectrics at Millimeter Wave Frequencies, IEEE Transactions on
Microwave Theory and Techniques, vol. MTT-28, No. 11, Nov. 1980,
pp. 1205-1212. cited by other .
Heshmatollah Maheri, Makoto Tsutsumi and Nobuaki Kumagai,
Experimental Studies on Magnetically Scannable Leaky-Wave Antennas
Having a Corrugated Ferrite Slab/Dielectric Layer Structure, IEEE
Transactions on Antennas And Propagation, vol. AP-36, No. 7, Jul.
1988, pp. 911-917. cited by other .
Orest Vendik, Igor Mironenko and Leon Ter-Martirosyan,
Superconductors Spur Application of Ferroelectric Films, Microwaves
and RF, vol. 33, No. 7, Jul. 1994, pp. 67-70. cited by other .
R. E. Horn, H. Jacobs, K. L. Klohn and E. Freibergs,
Single-Frequency Electronic-Modulated Analog Line Scanning Using a
Dielectric Antenna, IEEE Transactions on Microwave Theory and
Techniques, vol. MTT-30, No. 5, May 1982, pp. 816-820. cited by
other .
S. V. Zaitsev and A. T. Fialkovskii, Edge Effects in Strip
Structures With an Arbitrary Grazing Angle of The Wave. Waves in a
Microstrip Waveguide, Radio Phys. Quant. Electron. vol. 24, No. 9,
Sep., 1981, pp. 786-791. cited by other .
Wolfgang Menzel, A New Travelling-Wave Antenna in Microstrip, AEU,
vol. 33, No. 4, Apr. 1979, pp. 137-140. cited by other .
A. A. Oliner and K. S. Lee, The Nature of the Leakage from Higher
Modes on Microstrip Line, 1986 IEEE MTT-S Digest, Baltimore, MD,
Jun. 2-4, 986, pp. 57-60. cited by other.
|
Primary Examiner: Ho; Tan
Attorney, Agent or Firm: Fliesler Meyer LLP
Claims
In the claims:
1. A fixed-frequency beam-steerable leaky-wave microstrip antenna
comprising: a grounded element; a dielectric coupled to said
grounded element; and conducting traces coupled to the dielectric,
the conducing traces including: a pair of non-radiating conductive
elements; and a plurality of radiating strips, each of the
radiating strips connected between the pair of non-radiating
conductive elements, each of said plurality of radiating strips
including a center-loaded varying reactance element.
2. The fixed frequency beam steerable leaky wave microstrip antenna
of claim 1 wherein each of the varying reactance elements is a
variable capacitor.
3. The fixed frequency beam steerable leaky wave microstrip antenna
of claim 1 wherein each of the varying reactance elements is a
varactor diode.
4. The fixed frequency beam steerable leaky wave microstrip antenna
of claim 1 wherein the pair of non-radiating conductive elements
includes: a driving port having a first and second driving end, the
first driving end configured to receive a first driving signal, the
second driving end configured to receive a second driving signal,
the first signal being 180 degrees-out-of-phase with the second
driving signal; a terminating port having a first terminating end
and a second terminating end, the first terminating end connected
to a first resistive load, the second terminating end connected to
a second terminating load.
5. The fixed frequency beam steerable leaky wave microstrip antenna
of claim 4 further comprising: a biasing DC voltage source coupled
between the first terminating end and the second terminating
end.
6. The fixed frequency beam steerable leaky wave microstrip antenna
of claim 1 wherein each of the radiating strips has the same width,
length and inter-strip spacing.
7. A fixed frequency beam steerable leaky wave microstrip antenna,
comprising: a grounded element; a dielectric coupled to said
grounded element; and a pair of radiating strips coupled to said
dielectric, the pair of radiating strips separated by a generally
uniform gap and including: variable reactance elements mounted in
shunt across the gap, wherein the pair of radiating strips
includes: a driving port having a first and second driving end, the
first driving end configured to receive a first driving signal, the
second driving end configured to receive a second driving signal,
the first signal being 180 degrees-out-of-phase with the second
driving signal; a terminating port having a first terminating end
and a second terminating end, the first terminating end connected
to a first resistive load, the second terminating end connected to
a second terminating load.
8. A fixed-frequency beam-steerable leaky-wave microstrip antenna
comprising: a grounded element; a dielectric coupled to said
grounded element; and a pair of radiating strips coupled to said
dielectric, the pair of radiating strips separated by a generally
uniform gap and including: variable reactance elements mounted in
shunt across the gap, wherein the pair of radiating strips
includes: a driving port having a first and second driving end, the
first driving end configured to receive a first driving signal, the
second driving end configured to receive a second driving signal,
the first signal being 180 degrees-out-of-phase with the second
driving signal; a terminating port having a first terminating end
and a second terminating end, the first terminating end connected
to a first resistive load, the second terminating end connected to
a second terminating load; a biasing DC voltage source coupled
between the first terminating end and the second terminating
end.
9. A method for generating a fixed-frequency beam-steerable leaky
wave from a leaky wave microstrip antenna, comprising: providing
conducting traces coupled to a dielectric, the dielectric coupled
to a grounded element, the conducting traces including: a pair of
non-radiating conducting strips; and a plurality of radiating
strips, the plurality of radiating strips coupled between the pair
of non-radiating conducting strips, each of said plurality of
radiating strips including: a variable reactive-element having a
reactance value; driving the microstrip with a 180-degree hybrid
fixed-frequency signal, the signal configured to excite the micro
strip in a first higher order mode and configure the leaky wave
antenna to transmit a beam-steerable leaky wave; varying the
variable reactive-element reactance value to provide continuous
fixed frequency main beam steering.
10. The method of claim 9 wherein each of the variable
reactive-elements is center loaded on each of the plurality of
radiating strips.
11. The method of claim 9 wherein each of the variable
reactance-elements is a varactor diode.
12. The method of claim 9 wherein each of the plurality of
radiating strips is configured to have a substantially similar
length, width and inter-strip spacing.
13. A method for generating a fixed-frequency beam-steerable leaky
wave from a leaky-wave microstrip antenna, comprising: providing
conducting traces coupled to a dielectric, the dielectric coupled
to a grounded element, the conducting traces including: a pair of
radiating strips, the pair of radiating strips separated by a
generally uniform gap and including: variable reactance-elements
having a reactance value and mounted in shunt across the gap;
driving the radiating strips with a 180-degree-hybrid
fixed-frequency signal, the signal configured to excite the micro
strip in a first higher order mode and configure the leaky wave
antenna to transmit a beam steerable leaky wave; varying the
variable reactance-element reactance value to provide continuous
fixed-frequency main-beam steering.
14. The method of claim 13 wherein each of the variable
reactance-elements is a varactor diode.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is related to the following United States
Patents and Patent Applications, which patents/applications are
assigned to the owner of the present invention, and which
patents/applications are incorporated by reference herein in their
entirety: U.S. patent application Ser. No. 10/439,197, entitled
"LEAKY WAVE MICROSTRIP ANTENNA WITH A PRESCRIBABLE PATTERN", filed
on Apr. 15, 2003, now U.S. Pat. No. 6,839,030.
FIELD OF THE INVENTION
The current invention relates generally to fixed-frequency
beam-steerable leaky-wave antennas, and more particularly to
fixed-frequency beam-steerable leaky-wave microstrip antennas.
BACKGROUND OF THE INVENTION
Leaky-wave antennas are electromagnetic traveling-wave radiators
fed at one end and terminated in a resistive load at the other. The
feeding end is used to launch a wave that travels along the antenna
while leaking energy into free space. Power remaining in the
traveling wave is absorbed as it reaches the terminated end. The
fact that a single feed is used to excite a leaky-wave antenna
results in higher radiation efficiency in comparison with a
microstrip antenna array. In addition, a leaky-wave antenna does
not suffer from spurious-radiation and ohmic losses associated
usually with a corporate-fed microstrip array. The aforementioned
features of leaky-wave antennas make them well suited for operation
at high frequencies.
In 1979, a traveling-wave microstrip antenna based on the first
higher-order mode (EH.sub.1) in microstrip was first disclosed. A
microstrip is defined herein to be an electromagnetic waveguide
made up of conducting traces lying on the top surface of a
conductor-backed dielectric slab. The antenna was asymmetrically
fed by means of a microstrip line as shown in FIG. 1a, and
transverse slots located along the center line of the antenna were
used to suppress the fundamental mode. Using a quarter-wave
transformer, the input impedance of the antenna was matched to the
characteristic impedance of the microstrip feed line. The antenna
radiated an x-polarized main beam at an angle .theta. of
37.5.degree. away from broadside (the z direction). It exhibited an
impedance bandwidth broader than that of the resonant microstrip
patch, but also produced a high backlobe level.
It was later shown that the microstrip antenna introduced
previously could have been operated as a leaky-wave antenna had it
been made longer (4.85 times .lamda..sub.o long instead of 2.23
times .lamda..sub.o, where .lamda..sub.o is the free-space
wavelength at the design frequency). It was also shown that the
high backlobe level exhibited by the previous antenna is due to the
fact that 35% of the incident power is reflected at the terminated
end, with the backlobe appearing at the same angle as the main beam
when measured from broadside. A three-dimensional angled view of
the leaky-wave microstrip antenna is shown in FIG. 2.
The main-beam direction of a leaky-wave antenna scans well with
frequency. However, attempting to scan the same beam at fixed
frequency has so far been either impractical (for example, use of
liquid dielectric as disclosed in "Leaky-wave antennas using
artificial dielectrics at millimeter-wave frequencies", Bahl et
al., IEEE Transactions on Microwave Theory and Techniques, vol.
MTT-28, no. 11, pp.1205 1212, November 1980, or biased ferrite as
disclosed in "Experimental studies of magnetically scannable
leaky-wave antennas having a corrugated ferrite slab/kielectric
layer structure", Maheri et al., IEEE Transactions on Antennas and
Propagation, vol. AP-36, no7, pp. 911 917, July 1988), inefficient
(only 50% efficiency at 40 GHz, as disclosed in "Superconductors
spur application of ferroelectric films", Vendik et al., Microwaves
& RF, vol. 33, no. 7, pp. 67 70, July 1994), or did not provide
a large scan range (only 5.degree., as disclosed in
"Single-frequency electronic-modulated analog-line scanning using a
dielectric antenna", Horn et al., IEEE Transactions on Microwave
Theory and Techniques, vol. MTT-30, no. 5, pp. 816 820, May
1982).
In 1998, the leaky-wave microstrip antenna previously disclosed was
transformed into a periodic structure as shown in FIGS. 3, 4a and
4b, by Noujeim and Balmain, as discussed in K. M. Noujeim, "Fixed
Frequency beam-steerable leaky-wave antennas, "Ph. D. Thesis,
University of Toronto, Ontario, Canada, 1998, and K. M. Noujeim and
K. G. Balmain, "Fixed-frequency beam-steerable leaky-wave antennas,
"XXVIth General Assembly, International Union of Radio Science
(URSI), August 1999. Identical varactor diodes were used as
phase-shifting elements to series-connect the radiating rectangular
patches. Noujeim and Balmain showed that the main beam of the
resulting structure may be scanned continuously at fixed frequency
by varying the reverse-bias voltage across the varactor diodes from
0 to 900 volts. For a microstrip with a relative dielectric
permittivity of 6.15, they obtained a 60.degree. scan range both
theoretically and experimentally at a frequency f=5.2 GHz. Due to
the fact that the varactor diodes were arranged in series, the
maximum voltage required to reverse-bias them is high (900
volts).
Though fixed frequency leaky wave microstrip antennas have
developed over the years, there is still a need for better, more
efficient implementations. What is needed is a fixed frequency
beam-steerable leaky-wave microstrip antenna that improves over the
shortcomings and disadvantages over those of the prior art.
SUMMARY OF THE INVENTION
The present invention addresses the limitations and disadvantages
of the prior art by introducing a fixed-frequency continuously
beam-steerable leaky-wave antenna in microstrip. The antenna's
radiating strips are loaded with identical shunt-mounted
variable-reactance elements, resulting in low reverse-bias-voltage
requirements. The microstrip antenna is excited in its first
higher-order mode by means of two equal-amplitude and
180.degree.-out-of-phase signals. These signals are applied to the
feed end of the microstrip at two ports. The microstrip antenna
length is chosen such that more than 90% of the input power is
radiated by the electromagnetic wave by the time it reaches the
terminated antenna end. By varying the reverse-bias voltage across
the variable-reactance elements, the main beam of the antenna may
be scanned continuously at fixed frequency.
In one embodiment, the antenna consists of an array of radiating
strips. In this embodiment, each strip includes a
variable-reactance element. The variable-reactance element is
generally uniform throughout the microstrip. Changing the element's
reactance value has a similar effect as changing the length of the
radiating strips. This is accompanied by a change in the phase
velocity of the electromagnetic wave traveling along the antenna,
and results in continuous fixed-frequency main-beam steering.
In another embodiment, the antenna consists of two long radiating
strips separated by a small gap. In this embodiment,
variable-reactance elements are mounted in shunt across the gap at
regular intervals. In one embodiment, the variable-reactance
elements are about the same or identical. A continuous change in
the reactance value has a similar effect as changing continuously
the width of the radiating strips. This results in a continuous
change in the phase velocity of the electromagnetic wave traveling
along the antenna, thereby achieving continuous fixed-frequency
main-beam steering.
The variable-reactance elements can take the form of varactor
diodes, ferroelectric films such as BST (Barium Strontium
Titanate), or MEMS (Micro-Electro-Mechanical Systems)
varactors.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is an illustration of a top view of a traveling wave
microstrip antenna of the prior art.
FIG. 1b is an illustration of a side view of a traveling wave
microstrip antenna of the prior art.
FIG. 2 is an illustration of a microstrip leaky-wave antenna of the
prior art.
FIG. 3 is an illustration of a fixed-frequency beam-steerable
leaky-wave microstrip antenna of the prior art.
FIG. 4a is an illustration of a side view of a fixed-frequency
beam-steerable leaky-wave antenna of the prior art.
FIG. 4b is an illustration of a top view of a fixed-frequency
beam-steerable leaky-wave antenna of the prior art.
FIG. 5 is an illustration of a reactively loaded fixed-frequency
beam-steerable leaky-wave microstrip antenna in accordance with one
embodiment of the present invention.
FIG. 6 is an illustration of the top view of a reactively loaded
fixed-frequency beam-steerable leaky-wave microstrip antenna in
accordance with one embodiment of the present invention.
FIG. 7 is an illustration of an angled view of a reactively loaded
fixed-frequency beam-steerable leaky-wave microstrip antenna in
accordance with one embodiment of the present invention.
FIG. 8a is an illustration of a cross sectional view of a
reactively loaded fixed-frequency beam-steerable leaky-wave
microstrip antenna in accordance with one embodiment of the present
invention.
FIG. 8b is an illustration of a top view of a reactively loaded
fixed-frequency beam-steerable leaky-wave microstrip antenna in
accordance with one embodiment of the present invention.
FIG. 9 is an illustration of a transmission-line model for
transverse wave propagation in accordance with one embodiment of
the present invention.
FIG. 10 is a model for determining the open end impedance of the
leaky wave antenna in accordance with one embodiment of the present
invention.
FIG. 11 is an illustration of a plot of the normalized leakage
constant in a reactively loaded microstrip in accordance with one
embodiment of the present invention.
FIG. 12 is an illustration of a plot of the normalized phase
constant in a reactively loaded microstrip in accordance with one
embodiment of the present invention.
FIG. 13 is an illustration of a plot of the normalized H-plane
power gain pattern in a reactively loaded microstrip in accordance
with one embodiment of the present invention.
FIG. 14 is an illustration of a plot of the normalized leakage
constant in a reactively loaded microstrip in accordance with one
embodiment of the present invention.
FIG. 15 is an illustration of a plot of the normalized phase
constant in a reactively loaded microstrip in accordance with one
embodiment of the present invention.
FIG. 16 is an illustration of a plot of the normalized H-plane
power gain pattern in a reactively loaded microstrip in accordance
with one embodiment of the present invention.
DETAILED DESCRIPTION
The present invention discloses an improved fixed frequency
continuously beam-steerable leaky-wave antenna in microstrip. The
antenna's radiating strips are loaded with identical shunt-mounted
variable-reactance elements, resulting in low reverse-bias-voltage
requirements. The microstrip antenna is excited in its first
higher-order mode by means of two equal-amplitude and
180.degree.-out-of-phase signals. These signals are applied to the
feed end of the microstrip conducting traces at two ports. A port
is defined herein to consist of two closely spaced terminals across
which a signal may be applied. About ninety percent of the input
power is radiated by the electromagnetic wave by the time it
reaches the terminated antenna end. By varying the reverse-bias
voltage across the variable-reactance elements, the main beam of
the antenna may be scanned continuously at fixed frequency.
An angled three dimensional view of a reactively loaded fixed
frequency beam steerable leaky wave microstrip antenna 500 in
accordance with one embodiment of the present invention is
illustrated in FIG. 5. Leaky wave microstrip antenna 500 includes a
ground plane 510, a dielectric 520 coupled to the ground plane 510,
and a radiating strip 530 coupled to the dielectric 520. In one
embodiment, the ground plane and radiating strip are comprised of
copper. In the embodiment shown, the antenna consists of an array
of radiating strips. Each strip includes a variable-reactance
element. The variable-reactance element is generally uniform
throughout the microstrip. In one embodiment, the
variable-reactance elements can take the form of varactor diodes,
ferroelectric films such as BST (Barium Strontium Titanate), or
MEMS (Micro-Electro-Mechanical Systems) varactors. Changing the
element's reactance value has a similar effect as changing the
length of the radiating strips. This is accompanied by a change in
the phase velocity of the electromagnetic wave traveling along the
antenna, and results in continuous fixed-frequency main-beam
steering.
A top view of the reactively loaded fixed frequency beam steerable
leaky wave microstrip antenna of FIG. 5 is illustrated in FIG. 6 in
accordance with one embodiment of the present invention. Leaky wave
microstrip antenna 600 of FIG. 6 includes conducting traces coupled
to a dielectric 610. The conducting traces include a series of
radiating strips 620 placed between two non-radiating conducting
elements 625. Each of the radiating strips includes a
variable-reactance element 630. The microstrip is excited in its
first higher-order mode by means of two equal-amplitude and 180
degree-out-of-phase signals. These signals are applied to the feed
end of the conducting traces at the port as illustrated. In one
embodiment, the driving signals are provided by signal source 640.
DC block circuitry 650 may be implemented to block DC signals from
the signal source 640. The microstrip antenna is terminated with a
resistive load 660. A bias tee 670 and DC voltage source 680 are
provided at the terminating ends of the antenna as illustrated.
The length 1.sub.a of the microstrip antenna is chosen such that
more than ninety percent of the input power is radiated by the
electromagnetic wave when it reaches the terminated antenna end. In
one embodiment, this length is about 5.lamda., five times the free
space wavelength at the operating frequency. In one embodiment, the
length of the radiating strips 1.sub.s is about 0.45.lamda.g, 0.45
times the guide wavelength at the operating frequency. Thus, the
length of the non-radiating conducting elements 625 is about
1.sub.a. The width w.sub.a of the non-radiating conducting elements
is about the same. The width w.sub.s and inter-strip spacing d of
the radiating strips is generally uniform throughout the leaky wave
microstrip antenna.
Loading the strips with variable reactance elements affects the
phase of the wave traveling along the x direction, transverse to
the strips. In operation, the microstrip is driven by two
equal-amplitude and 180-degree-out-of-phase signals provided by
signal source 640. The printed microstrip feed points receive the
two signals having a 180 degree phase difference in order to excite
the first higher order mode in the microstrip. In one embodiment,
the DC block 650 is implemented to prevent DC signals from reaching
the signal source. In the embodiment shown, the DC block mechanisms
are implemented as capacitors.
The power from the two applied signals is radiated as the
electromagnetic wave travels along the microstrip antenna. As
mentioned above, the length of the microstrip antenna is chosen
such that approximately ninety percent of the wave power will be
radiated by the antenna structure as the wave travels along the
antenna. In one embodiment, a resistive load R.sub.L 660 is placed
at each terminating end of the microstrip to absorb the energy
remaining in the traveling wave as it reaches the antenna end.
In one embodiment, additional circuitry may be coupled to the
conducting traces to vary the reactance of the cell elements. For
purposes of discussion only, the cell elements will be considered
capacitors. In the embodiment shown, a DC voltage source 680 is
used to vary the voltage across the variable reactance elements,
capacitors, 630. As the capacitance is increased, the phase
velocity along the antenna is decreased. The decreased phase
velocity shifts the y-polarized main-beam maximum toward endfire,
closer to the x direction. As the capacitance is decreased, the
phase velocity along the antenna increases, thereby causing the
y-polarized main-bean maximum to shift toward-broadside, closer to
the z direction. In one embodiment, where DC voltage sources are
implemented, the conducting traces are coupled to bias tees 680 at
each terminating end. One purpose of the bias tees is to allow the
application of the DC bias required to control the variable
reactance element of each radiating strip, while preventing signal
power from reaching the DC source. The bias tees also prevent the
DC voltage from being applied to the load resistors 650.
A reactively loaded fixed-frequency beam-steerable leaky-wave
microstrip antenna in accordance with another embodiment of the
present invention is illustrated in FIGS. 7, 8a and 8b. Microstrip
antenna 700 of FIG. 7 includes a ground plane 710 coupled to
dielectric 720. Dielectric 720 is coupled to a loaded strip 730.
The loaded strip consists of a pair of radiating strips 740 and
variable reactive elements 750. The pair of radiating strips 740
include a driven end 760 and a terminated end 770. The variable
reactive elements consist of variable reactance elements placed in
shunt at regular intervals between the two radiating strips 740. In
one embodiment, the variable reactance elements may be about the
same or identical. In another embodiment, the variable reactance
elements may be substantially identical varactor diodes. In another
embodiment, the variable reactive elements can take the form of a
ferroelectric film such as Barium Strontium Titanate (BST) or
micro-electromechanical systems (MEMS) varactors placed in shunt at
regular intervals between the two radiating strips.
A cross sectional view of a reactively loaded fixed-frequency
beam-steerable leaky-wave microstrip antenna 800 is shown in FIG.
8a. The microstrip antenna 800 includes a conductor 810 coupled to
a dielectric 820. A pair of radiating strips 830 are coupled to
dielectric 820. The microstrip antenna is reactively loaded with
variable reactive loading elements 840 placed in shunt at regular
intervals between the two radiating strip. In the embodiment
illustrated in FIG. 8a, the variable reactive loading elements are
substantially identical varactor diodes. In another embodiment, the
reactive loading can take the form of a ferroelectric film such as
BST or MEMS varactors.
A top view of a reactively loaded fixed-frequency beam-steerable
leaky-wave microstrip antenna 850 is shown in FIG. 8b. Microstrip
antenna 850 includes dielectric 860 coupled to a pair of radiating
strips 870. Dielectric 860 is also coupled to a conducting ground
plane, though this plane is not shown in FIG. 8b. As illustrated in
FIG. 8b, the radiating strips are driven by a pair of
equal-amplitude and 180-degree-out-of-phase signals generated by
signal source 880. The signals travel through the microstrip
antenna while radiating energy into free space, reach the
terminating end, and are terminated by the termination resistance
885. The terminating end also includes bias tee circuitry 890 and a
DC voltage source 895 for biasing the reactive loading elements. As
in FIG. 8a, the microstrip antenna 850 of FIG. 8b is reactively
loaded with variable reactive elements placed in shunt at regular
intervals between the two radiating strips. In the embodiment
illustrated in FIG. 8b, the variable reactive elements are
identical varactor diodes. The length of the pair of radiating
strips is approximately five times the free space wavelength at the
operating frequency. The total width of the loaded radiating strips
is approximately 0.45 times the guide wavelength at the operating
frequency.
The leakage and propagation constants for the fixed frequency beam
steerable leaky wave microstrip antenna in the embodiment of the
present invention illustrated in FIGS. 7, 8a and 8b may be
calculated as discussed in reference to FIGS. 9 16. The values
calculated are intended as examples only, and the scope of the
present invention is not intended to be limited by the ranges
discussed. Rather, the discussion of calculations is intended to
enable the design of fixed-frequency beam-steerable leaky-wave
microstrip antennas for different applications.
As illustrated in FIG. 8a, a reactive sheet of width
.delta.<<h<<d, and surface reactance
X.sub.s=-1/(.omega.C) (.OMEGA./square) lies along the bisecting
line of the top 2d-wide conductor. Here, the dielectric thickness h
is chosen such that surface-wave modes beyond the TM.sub.0 mode are
cutoff.
The structure shown in FIG. 8a supports hybrid modes whose complex
propagation constants may be found by application of the
transverse-resonance technique disclosed in references including
"Microstrip leaky-wave antennas," A. A. Oliner and K. S. Lee, 1986
IEEE International Antennas and Propagation Symposium Digest,
Philadelphia, Pa., pp. 443 446, Jun. 8 13, 1986 (Oliner), "On field
representations in terms of leaky modes or eigenmodes," N.
Marcuvitz, IRE Transactions on Antennas and Propagation, vol. AP-4,
no. 3, pp. 192 194, July 1956, (Marcuvitz), and "Edge effects in
strip structures with an arbitrary grazing angle of the wave. Waves
in a microstrip waveguide," S. V. Zaitsev and A. T. Fialkovskii,
Radio Phys. Quant. Electron., vol. 24, no. 9, pp. 786 791,
September 1981 (Zaitsev), all of which are hereby incorporated by
reference. The first step of this technique is to predict Z.sub.h,
the impedance of the open end located at x=.+-.d.
The open-end impedance is found by making use of the
two-dimensional finite-difference time-domain (2D FDTD) technique
disclosed in "Numerical solution of initial boundary value problems
involving Maxwell's equations in isotropic media," K. S. Yee, IEEE
Transactions on Antennas and Propagation, vol. 14, pp. 302 307,
1966 (Yee), incorporated herein by reference, in which use is made
of a twelve-cell-thick perfectly matched layer (PML) as disclosed
in "A perfectly matched layer for the absorption of electromagnetic
waves," J.-P. Berenger, Journal of Computational Physics, vol. 114,
pp. 185 200, 1994, incorporated herein by reference, on the top,
left, and right walls as shown in FIG. 10. A y-polarized Gaussian
pulse generated by a voltage source located between the conducting
bottom wall and the top strip at x=x.sub.g is incident on the open
end. The ratio of the Fourier transforms of the y-polarized
electric field and z-polarized magnetic field at the open end
(x=x.sub.h) provides Z.sub.h.
The transverse-resonance technique may be applied to the circuit
shown in FIG. 9. This results in the following equation for the
complex propagation constant along the x direction:
.gamma..times..times..times..times..times..times..times..times..GAMMA..fu-
nction..function..tau..PHI..times..pi..times..times.
##EQU00001##
where n is the propagation-mode index, .GAMMA. is the reflection
coefficient, Z.sub.0 is the TEM wave impedance in a dielectric
having a relative constant .epsilon..sub.r, .phi.=Arg(.GAMMA.(d)),
and: .tau..function..times..times..times..times. ##EQU00002##
With .gamma..sub.x known, the complex propagation constant
.gamma..sub.Z along the direction of wave propagation may be
calculated readily using: .gamma..sub.Z= {square root over
(k.sub.s.sup.2-.gamma..sub.x.sup.2)} (2)
where k.sub.s is the propagation constant of the TM.sub.0
surface-wave mode, assumed by a proper choice of h to be the only
propagating mode. Eqs. (1) and (2) show the dependence of
.gamma..sub.z on the surface reactance X.sub.s, and thus on the
reactive loading. The extent of this dependence and its
implications will now be illustrated by two antenna examples.
The values of .gamma..sub.z and .gamma..sub.x can be used to
calculate normalized values for the leakage and propagation
constants of the EH.sub.1 mode propagating along a reactively
loaded microstrip. The results are shown in FIGS. 11 and 12 for
different values of C ranging from 0.05 1.0 pF. For the data
plotted in FIGS. 11 and 12, the microstrip dielectric constant
.epsilon..sub.r=2.2, the dielectric thickness h=0.127 mm, and the
strip width 2 d=3.5 mm. FIG. 11 includes plots for values of C at
1.0 pf at 1150, 0.2 pf at 1140, 0.1 pf at 1130, 0.07 pf at 1120,
and 0.05 pf at 1110.
FIG. 12 shows that an increasing value of C has the effect of
making the microstrip waveguide appear wider, and causes a downward
shift in the cutoff frequency of the EH.sub.1 mode. Here, a shift
of about 3 GHz in the cutoff frequency of the EH.sub.1 mode is
observed as C is increased from 0.05 to 1 pF. In particular, FIG.
12 illustrates plots of C at 1.0 pf at 1210, 0.2 pf at 1220, 0.1 pf
at 1230, 0.07 pf at 1240, 0.05 pf at 1250. FIG. 12 also shows that
at a constant frequency f, a continuous increase in the value of C
is accompanied by a continuous decrease in the phase velocity along
the microstrip, and thus a continuous movement of the main-beam
maximum toward endfire.
For a microstrip of length L, the H-plane power-gain pattern may be
calculated by treating the microstrip as a line source as discussed
in Antenna Theory and Design, John Wiley & Sons, Inc., W. L.
Stutzman and G. A. Thiele, 605 Third Ave., New York, N.Y.
10158-0012, pp. 137 141 and 173 174, 1981 (Stutzman), incorporated
herein by reference, and by making use of the element factor of an
x-directed infinitesimal current element lying on a grounded
dielectric slab of infinite extent as discussed in "Electric
surface current model for the analysis of microstrip antennas with
application to rectangular elements," P. Perlmutter, S. Shtrikman,
and D. Treves, IEEE Transactions on Antennas and Propagation, vol.
AP-33, no. 3, pp. 301 311, March 1985 (Perlmutter), also
incorporated herein by reference. For a microstrip of length L=4.9
.lamda..sub.0, where .lamda..sub.0 is the free-space wavelength at
f=30 GHz, this approach results in the normalized H-plane
power-gain patterns shown in FIG. 13 for different values of C
ranging from 0.05 1.0 pF. This choice of L ensures that at least
90% of the input power is radiated by the time the EH.sub.1 wave
reaches the end of the microstrip.
FIG. 13 illustrates the normalized H-plane power pattern for a
microstrip excited at f=30 GHz for a microstrip that is 4.9
.lamda..sub.0 in length and has dielectric constant
.epsilon..sub.r=2.2. The normalization factor for each of the power
patterns is its maximum power gain. FIG. 16 shows that as C is
decreased from 1 to 0.05 pF, the main-beam maximum scans a
35-degree range at a constant frequency f=30 GHz. This is
accompanied by a widening of the main beam, and is due mainly to
the fact that the leakage constant a shown in FIG. 14 increases as
C is decreased, resulting in a shorter radiating aperture.
An analysis similar to that performed above may also be applied to
a microstrip with a dielectric constant .epsilon..sub.r=3.78,
thickness h=0.127 mm, and strip width 2 d=2.67 mm. The results are
shown in FIGS. 14, 15, and 16 for values of C ranging from 1.0 to
0.05 pF. In this illustrative case, a 64 degree main-beam scan
range is achieved at a constant frequency f=30 GHz, and is
accompanied by a shift of about 4 GHz in the cutoff frequency of
the EH.sub.1 mode.
The reactive loading implemented in the cells comprising the
microstrip can take a variety of forms. In one embodiment, the
reactive loading may include a ferroelectric film such as BST, as
disclosed in "Superconductors spur application of ferroelectric
films," O. Vendik, I. Mironenko, and L. Ter-Martirosyan, Microwaves
& RF, vol. 33, no. 7, pp. 67 70, July 1994, and incorporated
herein by reference. Alternatively, a periodic array of
ferroelectric strips placed in shunt across the microstrip center
gap can be used, and would result in antennas with a higher
radiation efficiency. Another form of loading is a periodic array
of varactors (Schottky or MEMS, as disclosed in "Distributed MEMS
true-time delay phase shifters and wideband switches," N. S.
Barker, and G. M. Rebeiz, IEEE Transactions on Microwave Theory and
Techniques, vol. 46, no. 11, November 1998, and incorporated herein
by reference) requiring a reverse-bias voltage range that is much
smaller than that used in a microstrip implementation wherein
elements are interconnected using varactor diodes, due to the shunt
mounting of the varactors across the microstrip center gap. Other
types of loading implementations using alternative varying reactive
elements are considered within the scope of the present
invention.
The phase velocity along a reactively loaded microstrip operating
in its first higher-order mode may be varied continuously at
constant frequency by varying its surface reactance. This effect
can be used to achieve fixed-frequency continuous main-beam
steering. It was also found that a change in the surface reactance
is accompanied by a shift in the cutoff frequency of the first
higher-order mode. This effect is similar to changing the width of
the microstrip waveguide, and may be used in the design of antennas
with a continuously adjustable operating frequency range. The
reactively loaded microstrip may also be used as a variable-delay
transmission line when operated below f.sub.c1, the cutoff
frequency of its first higher-order mode. On the other hand, when
loaded periodically with reverse-biased Schottky varactors, and
driven in large-signal mode at frequencies that are much smaller
than f.sub.c1 the structure may be used as a nonlinear transmission
line for the generation of nonlinear waves such as electrical shock
waves and solitons as disclosed in "Active and nonlinear wave
propagation devices in ultra fast electronics and optoelectronics,"
M. J. W. Rodwell et al., IEEE Proceedings, vol. 82, no. 7, pp.1037
1058, July 1994, and herein incorporated by reference.
Other features, aspects and objects of the invention can be
obtained from a review of the figures and the claims. It is to be
understood that other embodiments of the invention can be developed
and fall within the spirit and scope of the invention and
claims.
The foregoing description of preferred embodiments of the present
invention has been provided for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Obviously, many
modifications and variations will be apparent to the practitioner
skilled in the art. The embodiments were chosen and described in
order to best explain the principles of the invention and its
practical application, thereby enabling others skilled in the art
to understand the invention for various embodiments and with
various modifications that are suited to the particular use
contemplated. It is intended that the scope of the invention be
defined by the following claims and their equivalence.
* * * * *